Depends to an extent on how far away the viewer is going it be from the sign. You could try printing some letters of various sizes, placing them at the appropriate distance and seeing how easily they can be read and scaling your sign based on that. I will also depends on how well lit your sign will be, what font, etc.
I have two signs that I made, one is backlit white lettering on red background. The letters are 4.5" high and can be easily read from at least 30 feet way at night. The other sign is front lit with 8" white lettering on a green background but the light isn't very intense because scatter would pollute the display elements nearby and I'd say it is easily readable from about the same distance as the other sign. Both of the signs are only about 6' from the curb.
Greg